Luna's eyebrows shot up, “How? With what? The sun's 400 times larger than the moon, and even if I could give you a tow, it'd take me 8 light minutes to get there! And besides, I may have lowered it and put a supercharger in, but I'd never have enough grunt to tow it! Sorry, sis, but you're on your own. You can fix it, can't you?”

I tried the usual (Wiktionary, Merriam-Webster, The Free, Dictionary.com) and none of the definitions (foot soldier, sound, menial worker, fish, dessert), seem to match this kind of usage. What does grunt in this context mean?